As Leo's replacement Thomas Chamberlain arrives Nikki investigates the murders of Rachel Freedman, shot in the head, and her little autistic son Nathan, who was poisoned. Nikki rejects policewoman Anne Burchett's theory that Rachel killed them both but is perturbed when an impostor, claiming to be Rachel's sister, visits the morgue. At the same time the disfigured corpse of Eva Liron is discovered, killed with the same poison as Nathan. She had been the nanny to French hot shot footballer Isaac Dreyfus and was planning to blackmail him over a sex tape of them both. Jewish Isaac is a controversial figure due to his anti-Muslim sentiments but he denies ant part in the murders. Then another victim, Isaac's agent Mousa, is discovered dead.

The body of teenager Katie Bowman is found dumped in an open grave. DI Carter suspects Ryan Kelvin, boyfriend of Lizzie Kennedy, whose daughter Hannah was Katie's best friend. He argued with the deceased girl the day she died and has no alibi. Ryan is also Jack's half-brother, whom he has not seen since he was forced to testify against him in a court case some years earlier. To avoid a clash of interests Thomas brings in pathologist Helen Ferguson, suspending Jack. Jack however does his own sleuthing. showing Carter that Katie had been to a club where she met Christy Nash, a married doctor with whom she had sex. Nash's wife gives him a false alibi to protect their daughter but declares her intention to divorce him. Ryan is bailed and moves in with Jack, who now doubts his brother's innocence whilst Hannah is killed after confronting Dr Nash.

Things look bad for Ryan when Lizzie tells Carter that he rowed with Hannah a couple of hours before she died as a result of which he makes an unsuccessful suicide bid. Nikki is surprised at the lengths Jack will go to exonerate his brother until he tells her that was the assailant in the criminal case and Ryan took the blame to save his career. Jack pursues Christy, who admits to falling for Katie, unaware that she was a minor but some while afterwards Jack is the victim of a hit and run, which he survives. Ultimately the killer proves to be protecting a professional, rather than a personal, interest though Ryan, cleared of the murders, is not wholly innocent of another crime.

Three people, the Lakhanis, an Indian couple, and an employee are shot dead by a long range sniper on a garage forecourt - a seemingly motiveless crime. Ten miles away another victim, profoundly deaf Hannah Smithson, is also shot, though her boyfriend's brother Chris, targeted with her, survives but is unable to help with any information. Inspector Jane De Freitas, on the rebound from an affair with colleague Robert Drake, calls in the pathology team but with the sniper at large and shooting indiscriminately Thomas is concerned for ex-wife Julia and their daughter Rosie, now living with former drug user Conrad Devenish. After a fifth victim, Martin Cross,a charity worker at a school is shot the killer rings Jane to tell her that he is unstoppable.

Whilst the sniper claims another body, a French lorry driver, there are fears that he may next target a school and Thomas rushes to ensure his daughter's safety. Drake calls a public meeting to pass on police progress, whereat Martin Cross's brother Ian praises him. Nikki however believes that Cross may have been the only intended victim with the others slaughtered to give the impression of a serial killer. Jack learns that Cross regularly assaulted his wife and teen-age son Craig, who hated his father, and Craig has bullets in his bedroom. Nikki and Jane now feel that Craig was working in tandem with the murderer but when the sniper strikes again Craig is standing next to the victim. They start to wonder if Martin Cross was the only person the sniper wanted out of the way and how many were working with the killer.

Nikki and Jack are called to a tube station where a homeless ex-soldier has fallen under the wheels of a subway train. Leaving the station hotel receptionist Lana Sutherland meets another homeless man, Owen Hanmore, whom she invites to stay with her, becoming his lover. Meanwhile a series of murders occurs with no visible link, beyond the fact that all the victims had used the same tube station.

A shootout at a City hotel leaves young Ukrainian nanny Sofia Teplov dead, along with an unknown hitman. The Russian oligarch target Maksim Bazhanov narrowly escapes.

A young woman working for the Doshi property moguls, Amy Greenwood, is found dead in Maksim Bazhanov's apartment, and Maksim is discovered to have been in a relationship with Yasmin Doshi.

Whilst cage-fighting Jack takes on young Dean Fallon, drug dealer and son of career criminal Terry Fallon. When policeman Sam Honeywell's corpse is discovered in his burnt out car Dean is the main suspect after Honeywell had arrested him but is released for lack of evidence. Sam's colleague Carl Parry is then arrested when it is made known that Carl's wife Kate was having an affair with the dead man. After Dean has fallen out with his friend Jason Simons over Dean's girlfriend Tess Hughes Tess finds drugs hidden in her father Stan's garage, which she takes to Jason. Soon afterwards he too is killed.

The accidental fatal shooting of a Polish teenager during an armed police operation prompts a series of murders linking Turkish gangsters, the grieving family and a fractured firearms unit.

When the body of an armed police officer is found, the team join the search for the killer. Jack discovers a link to London's gun trade and puts himself in danger to discover the truth about the death of the young Pole.

As DCI Butcher moves closer to making an arrest, the team continue to pursue their own lines of inquiry. Amy's past begins to catch up with her and converge with the team's investigation.

When the body of a teenager is found in a field near the village where he lived the team must determine if the killer is a stranger or a local.